[PATCH] arm64: entry: Simplify tramp_alias macro and tramp_exit routine

[Will Deacon]

Hey Ard,

On Tue, Mar 07, 2023 at 12:57:27PM +0100, Ard Biesheuvel wrote:
> The tramp_alias macro constructs the virtual alias of a symbol in the
> trampoline text mapping, based on its kernel text address, and does so
> in a way that is more convoluted than necessary. So let's simplify it.
> 
> Also, now that the address of the vector table is kept in a per-CPU
> variable, there is no need to defer the load and the assignment of
> VBAR_EL1 to tramp_exit(). Given that tramp_alias no longer needs a temp
> register, this means we can restore X30 earlier as well, and only leave
> X29 for tramp_exit() to restore.
> 
> Finally, merge the native and compat versions of tramp_exit() - the only
> difference is whether X29 is reloaded from FAR_EL1, and we can just zero
> it conditionally rather than having two distinct code paths.
> 
> While at it, give some related symbols static linkage, considering that
> they are only referenced from the object file that defines them.
> 
> Cc: James Morse <james.morse at arm.com>
> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
> ---
>  arch/arm64/kernel/entry.S | 58 ++++++++------------
>  1 file changed, 22 insertions(+), 36 deletions(-)

[...]

> @@ -768,7 +753,7 @@ alternative_else_nop_endif
>   */
>  	.pushsection ".entry.tramp.text", "ax"
>  	.align	11
> -SYM_CODE_START_NOALIGN(tramp_vectors)
> +SYM_CODE_START_LOCAL_NOALIGN(tramp_vectors)
>  #ifdef CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY
>  	generate_tramp_vector	kpti=1, bhb=BHB_MITIGATION_LOOP
>  	generate_tramp_vector	kpti=1, bhb=BHB_MITIGATION_FW
> @@ -777,13 +762,14 @@ SYM_CODE_START_NOALIGN(tramp_vectors)
>  	generate_tramp_vector	kpti=1, bhb=BHB_MITIGATION_NONE
>  SYM_CODE_END(tramp_vectors)
>  
> -SYM_CODE_START(tramp_exit_native)
> -	tramp_exit
> -SYM_CODE_END(tramp_exit_native)
> -
> -SYM_CODE_START(tramp_exit_compat)
> -	tramp_exit	32
> -SYM_CODE_END(tramp_exit_compat)
> +SYM_CODE_START_LOCAL(tramp_exit)
> +	// Entered with Z flag set if task is native
> +	tramp_unmap_kernel	x29
> +	mrs	x29, far_el1			// restore x29
> +	csel	x29, x29, xzr, eq		// clear x29 for compat

Why do we care about zeroing x29 for a compat task?

Will